From these numbers I would say that the Texans and Vikings have played against slightly worse run defenses than the Titans. Now let's look at their own team's run blocking rank according to the PFF guys.

Run block team rank
Texans 2
Titans 28
Vikings 24

Here we see a huge difference!! The Texans have one the best run blocking teams in the league compared to the Titans and Vikings who are in the bottom eight. Therefore Foster is being helped a great deal by his own team's blocking.